[PATCH] D93838: [SCCP] Add Function Specialization pass

[Sjoerd Meijer via Phabricator via llvm-commits]

SjoerdMeijer added a comment.

One more data point.

MCF is one end of the spectrum with a LTO compile/link step of ~1 second, and SQLite somewhere at the other end with a compile time of 160 seconds (i.e., the amalgamation version: 184K lines of code (one file), over 6.4 megabytes in size). It specialises 4 functions, and thus triggers 4 times. The overhead of function specialisation is just 1.3% - 1.8%, and is mainly just in running the pass itself, not any knock on effect of the rest of the pipeline chewing on more instructions. I think this shows that:

- the cost of function specialisation is amortised with larger compile times,
- but of course also the ratio of the number of existing and new functions/instructions is important.

Similarly zooming in on the compile-times for SQLite:

| Timing report (in seconds)           | -O3      | -O3 + func spec |
| ------------------------------------ | -------- | --------------- |
| Pass execution 1                     | 159.9010 | 164.3134        |  |
| Register Allocation                  | 2.7357   | 2.7376          |  |
| Instruction Selection and Scheduling | 9.0910   | 9.0746          |  |
| Pass execution 2                     | 61.8962  | 61.6610         |  |
| DWARF Emission                       | 12.2326  | 12.2232         |  |
|

And again the function specialisation pass does not enter the top 5 of the most compile time consuming passes:

  2.8416 (  1.8%)   0.3040 (  5.6%)   3.1456 (  1.9%)   3.1457 (  1.9%)  FunctionSpecializationPass

The binary size is 1.575.571 bytes vs. 1.575.299 bytes, so slightly reduced which is interesting.

So, as it stands, and sort of repeating what I said earlier today: this looks like an good candidate for an initial commit. I.e., this looks like a good basis that performs the transformation, but we know this needs further cost-model tuning. For example, the first thing I would like to add is a budget to not let the functions grow out of proportion. What do we think about that?


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https://reviews.llvm.org/D93838